Factory-formed building products are used in the construction of buildings and other infrastructure. Examples include masonry units made from vitrified or non-vitrified clay such as bricks and roof tiles, and products made from Portland cement, lime, gypsum, and aggregates in the shape of blocks, sheets, bricks, and so forth. Manufacture of factory formed building products usually requires energy and generates greenhouse gases.
In an effort to reduce the total energy and greenhouse-gas footprint from manufacturing, conserve virgin materials, reduce unit cost, and/or improve the technical properties of these products, it has become more common to recycle post-industrial waste or byproducts as partial replacement for certain ingredients. Even with these recycled materials, these products continue to rely to a large degree on traditional energy- and carbon-intensive materials or methods to realize their performance characteristics.
Fly ash is a useful post-industrial by-product used as a partial replacement in cementitious products. The ASTM C618 specification refers to Classes of fly ash, termed Class F fly ash and Class C fly ash, distinguished by the coal from which they originate and the ultimate chemical composition of the ash. The Class F fly ashes may be generally obtained during the burning of anthracite or bituminous coal, and rarely manifest cementitious properties and hardening when mixed with water. Prior art generally relies on the use of fly ash in conjunction with another hydraulic cement (e.g., lime or Portland cement). Class F fly ash can act as a pozzolan in hydraulic cementing systems, a material capable of reacting with lime in the presence of water to form hydraulic setting compounds with cementing properties. Lime may be, for example, added or obtained as an in-situ product of the reaction between Portland cement and water. In the absence of lime, Class F fly ash may be incapable of significant reaction with water. This incapability of Class F fly ash to react with water could be attributed to the low presence of calcium either in the direct oxide or complex oxide form within the fly ash. A calcium component of fly ash may be necessary for creation of reaction products capable of providing cementing properties. On the other hand, Class C fly ash, normally produced from the burning of sub-bituminous or lignite coal, usually demonstrates cementitious properties in addition to pozzolanic properties generally attributed to its higher free lime and calcium compound content. Class C fly ash also may contain relatively larger amounts of calcium, both in direct oxide and complex oxide form, when compared to Class F fly ash. This calcium content may permit Class C fly ash to react with water without need for the presence of additional chemicals, and to generate reaction products capable with cementing properties. Thus, Class C fly ash may be used as a cementing material without added lime or Portland cement.
However, this opportunity of using Class C fly ash as a cementing material capable of reacting with water on its own without using additional cementing compounds has not been thoroughly exploited by current industry. Class C fly is usually obtained as a by-product of coal burning. Class C fly ash obtained this way is dependent on the naturally variable chemical composition of the coal. Utilities, such as power plants that burn sub-bituminous and lignitic coal to produce Class C fly ash, often do not regulate the quality of the ash produced with respect to chemical composition and physical properties. Rather, the ash produced is generally used as an as-is by-product. Such a by-product may be highly variable when compared to specially controlled manufactured products such as Portland cement and lime. These and similar aspects of Class C fly ash have restricted the use of Class C fly ash as the main or sole binder in products made with hydraulic cements.